Magneto-optical recording typically requires three disk revolutions to write new information on the disk. The first revolution is used to erase the media, the second, to write the information and the third, to read and verify that the write operation successfully recorded the data.
Various attempts have been made to reduce the overhead associated with an extra disk revolution for the read verify operation. U.S. Pat. No. 4,583,210, relates to an optical disk system where information is read back immediately after it is written. Two separate laser generators are provided; one for writing data and one for reading the data back.
U.S. Pat. No. 4,949,311, relates to an optical disk system with a single laser source to write on an optical disk and obtain a direct read-after-write. In order to accomplish the write and the read, a grate is used to split the laser beam into two beams. The first beam is focused onto the recording surface to generate a write spot, and the second beam is focused onto the recording surface to read back what has been written. The read spot is positioned some distance behind the write spot on the moving magnetic media; in essence, this approach involves the use of two laser beams although only one laser source.
Optical disk media is reactive to bursts of light such as can be produced by the rapid switching of a semiconductor laser. In order to write data on optical media, laser power is controlled to a fairly high power level in order that the media is heated and altered in accordance with an input data stream. In reading the data back, the laser power level is controlled to a lower level so that the media is not altered by the laser beam, but the reflected light indicates the presence or absence of media alterations in accordance with the recorded input data stream.
One type of erasable optical media is magneto-optic (MO) media, which is not permanently altered when data is written. In MO media, the magnetic orientation of the reactive material is altered in the writing process, and in the reading process the magnetic orientation is sensed. In writing data, the orientation of magnetic domains within a written mark on the media may take a north pole orientation to indicate a "0" digit, for example, while the magnetic orientation of the domain within a mark to indicate a "1" digit takes a south pole orientation. In the erasing process, the magnetic orientation is ordered such that the entire media takes the same orientation, for example, a north pole or "0" orientation.
When reading data from an MO optical disk, the remanent magnetization of one or the other polarity causes an angular displacement in the linear polarization of a reflected light beam according to the Kerr effect. The direction of angular displacement is dependent upon the north pole or south pole orientation of the media. When the reflected beam is split into its P and S polarization components, direction of the angular displacement of the light beam can be detected to produce an MO data signal in accordance with the recorded data.
In thermo-magnetic recording, a laser beam is focused to a spot on the surface of the optical disk. Heating of the reactive material of the disk results in a temporary softening of the remanent magnetization quality of the magnetic medium, so that through the application of an external magnetic field, the direction of magnetization can be set to either a north or a south polarity within the heated material. When the laser spot is removed, the medium cools so that the magnetic domains under the spot take either a north or a south polarity depending upon the direction of the external magnetic field applied during the heating process.
In order to write a succession of marks on MO media, the MO disk is moved at a constant velocity under the focused laser spot. During writing of a mark, the laser is energized to a relatively high level often accomplished through short, high power pulses interspersed with relatively long periods at the low power read level.